This invention relates to a photo enhanced CVD apparatus for depositing a ceramic coat or the like on a substrate.
For photo enhanced CVD, high pressure mercury lamps, and low pressure mercury lamps are used. The low pressure mercury lamp comprises a bulb for light emitting which is filled with argon gas at several Torr as a initiator besides mercury gas. The bulb is provide with a pair of inside electrodes which are connected with external contacts for inputting electric power. By virtue of high voltage applied to the electrodes, e.g., a commercial electricity supply, discharge takes place and mercury atoms are excited to the high energy levels, 6.sup.3 O.sub.0, 6.sup.3 P.sub.1, 6.sup.3 P.sub.2, and 6.sup.1 P.sub.1, as shown in FIG. 1. Out of the energy level transitions to the ground level of electron of the excited atoms, the transition from 6.sup.3 P.sub.1 is most likely in virtue of the collision with argon atoms, and therefore light with 254 nm in wavelength is emitted accompanying this transition. The spectral distribution of such a conventional mercury lamp is shown in FIG. 2. As seen from the diagram, the emission intensity is most strong at 254 nm in wavelength and the next is at near 185 nm.
On the other hand, in the current developing manufacturing method for forming a semiconductor film by photo enhanced CVD, an illumination of short wavelength, particularly of 185 nm, is effective for decomposing silane gas (Si.sub.n H.sub.2n+2, where n is an integer). So it has been demanded to obtain ultraviolet light intensified at a wavelength near 185 nm.
Further, because ultraviolet light is radiated through a light window from a mercury lamp in a prior art CVD apparatus, the deposition simultaneously takes place on the surface of the window as the radiation passes therethrough during deposition on a substrate, and eventually the optical reaction terminates when the product deposited on the window forms an opaque layer so thick that the ultraviolet light passing through the window is too diminished to excite the reaction. For this reason, there is a limitation to the thickness of a layer obtained by the deposition, and it is impossible to deposit an overlying layer continuously after the deposition of an underlying layer.